X11 driver for Postscript interpreter

[Jean Marie Diaz]

[These quotes are from an article posted to comp.sources.bugs.  I'm
crossposting to comp.windows.x because I'd like feedback from there as
well...]

   weissman = Terry Weissman of DEC. (weissman at decwrl.dec.com)
   caag = Crispin Goswell of RAL (caag at vd.rl.ac.uk)

   [My X11 driver] doesn't understand expose events.  If you obscure or
   iconify the postscript window, that data won't ever be repainted.  This
   is partially because I didn't see any straightforward way to put in an
   event-handling loop, and mostly because I was too lazy.  - Weismann

   [There *is* no straight-forward way of putting in an event-handling loop.
    PostScript is not designed to handle asynchronous input.
    I believe the only correct solution to this is to demand backing store
    from the X server. Some PostScript images take minutes or hours to
    compute. Requiring this to happen just because the user removes an
    obscuring window is just *wrong*. I think you've done the right
    thing by not attempting to fix this one. - caag]

Well, I wouldn't call it straightforward, but there is a way, which is
used by the X11 version of gnuplot (done by Paul Rubin here at Athena).
Gnuplot, in its interactive mode, is similar to xps (for lack of a
better abbreviation) in that it has an interpreter whose output goes to
a different ("output-only") window.

Paul's method is as follows:
1) Set up a signal handler which will send us a SIGIO interrupt when data
appears on the X server socket.
2) When we get this signal, disable the interrupts (because we're going to
start doing Xlib calls, which are not necessarily reentrant).
3) Update the screen.
4) Turn the handler back on.

This method works beautifully in gnuplot, which simply replots its
equation(s) over the exposed portion of screen.  Since this won't work
very well for xps, we have one final wrinkle: a call to
NewWindowHardware() creates both a Window, and a Pixmap the size and
color of the window.  Thereafter, whenever the window is written into,
the same operation is performed on the pixmap.  When we need to refresh,
we just copy from the appropriate area on the pixmap.

Comments?

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